Photovoltaic plants for generating electric power include a solar inverter comprising for example a semiconductor bridge converting the direct voltage generated by a solar generator into an alternating voltage.
In the solar inverters, when the bridge is being synchronized, the full voltage of the solar generator is applied to the individual semiconductors. When the solar inverter is ideally fed without interruption over the entire day, the semiconductors could be set to a voltage referred to as an MPP voltage (Maximum Power Point) of the solar generator. The MPP voltage is the voltage at a power peak of the generator characteristic. Often however, the supply is disrupted because of mains failure or of other reasons. In order for the solar inverter to be capable of resuming operation after such interruptions, it must be synchronized at the idle voltage of the generator, which in this case is also applied at the individual semiconductors. Depending on the type of the generator, this idle voltage may exceed the MPP voltage by 25% and more. Therefore, the individual semiconductors must be devised for this significantly higher voltage. However, higher voltage semiconductors involve higher conduction losses and, as a result thereof, reduced efficiency of the inverter. If the inverter also comprises a buffer capacitor, the latter must also be devised for a higher voltage. The buffer capacitor is not only larger; it also is significantly more expensive.
Existing mounting regulations often only allow for determined maximum voltages. In the US for example, the solar plants are not allowed to have a voltage in excess of 600 V. Since breakdowns can never be excluded during supply, the generators of solar plants are generally dimensioned so that their idle voltage will not exceed the threshold of 600 V.
The document DE-A-40 41 672 shows and describes a protection device with a monitoring equipment for voltage increase due to failures for photovoltaic power generating plants, a current flow being interrupted by a line segment. The protection device comprises a short-circuit current circuit with an electrically actuatable switch.
In another document, namely the document JP 2004 254447 A, there is disclosed an inverter for a photovoltaic generator. There is provided that the DC side be short-circuited. A buffer capacitor is thereby provided on the DC side, as well as three diodes. The diodes are disposed directly behind the generator so that the buffer capacitor is immediately discharged when the DC side is short-circuited.
The document JP 2004 070709 A describes a protection circuit with a shorted circuit. A shunt is inserted to limit the voltage.
From the document DE 37 25 476 A1 there is also known a circuit arrangement for feeding an electric load from an electric power source with fluctuating efficiency. The circuit arrangement serves to feed an electric load from a solar generator. A capacitor 2 is located parallel to the solar generator. The control is such that, when the voltages are less than a prescribed minimum value, which is needed for secure operation of the supply apparatus, an initial load is connected to the solar generator and that, when the voltages increase over prescribed minimum value, an auxiliary voltage input of the supply apparatus is connected to the solar generator.
The printed document JP 11 312022 A shows and describes an inverter that is intended to be utilized for photovoltaic generators. The circuit comprises a coil, a switch element for short-circuiting the coil, a diode and behind the diode a capacitor. There is further provided a chopper circuit the function of which is to be discontinued when the generator voltages are too high.
A photovoltaic power generator apparatus for protecting a power inverter against overvoltage is known from the document EP 1 039 621 B1. In this system, a direct current of the solar generator is converted into an alternating current. The document discusses the issue that a conventional power inverter will not start immediately upon receiving power generated in the solar cell apparatus but only after a predetermined length of time after the output from the solar cell apparatus has been checked. This is carried out because self-testing of the power inverter takes a certain time. Since the conventional power inverter starts later as a result thereof, the open voltage at the solar generator increases. As a result, the semiconductors of the conventional power inverter must be devised for a voltage that is higher than the operating voltage. The result is a poorer operating efficiency. Therefore, the document EP 1 039 621 B1 suggests a circuit that comprises a switch element with a compensating resistor. This is intended to prevent the voltage from increasing. The compensating resistor in the circuit is set so that the generator voltage will not exceed the maximum operating voltage of the power inverter so that the semiconductors of the power inverter only need to be set to the maximum operating voltage. The circuit comprises a plurality of transistors, the compensating resistor being connected in parallel with one of the transistors. According to the document EP 1 039 621 B1, a parallel branch is virtually connected to the generator, said parallel branch tapping a defined working current from the generator, thus preventing idle voltage from occurring at the generator terminals. This branch is only shut down when a certain minimum current is tapped by the inverter, thus ensuring that the idle voltage will no longer be achieved, without the help of the additional branch either. The disadvantage of this method however is that the transverse branch remains active for a while when the power supplied is small so that there is power, which is not used. Further, the transverse branch must be well tuned to the connected solar generator in order to efficiently limit the voltage. If an inverter is intended to work flexibly with different types of modules and string currents, the transverse branch must always be set to the maximum current that it is possible to achieve, this involving that the losses were to even further increase. If the inverter is to resume operation after a failure, the compensating resistor must even be set to the full power of the photovoltaic generator.
From the DE 103 12 921 A1 there is known a circuit arrangement for using a direct voltage generated by at least one solar generator. The circuit arrangement comprises a capacitor and an inverter circuit connected downstream of the capacitor. With this circuit arrangement, it is possible to use commercially available inverters with potential isolation also for thin-film solar cells and to thus achieve improved efficiency. With this solution, a switch element is connected upstream of the capacitor. The switch element is connected in series with the generator and with a semiconductor bridge between the generator and the semiconductor bridge. The circuit arrangement is responsible for opening the switch element when a defined first generator voltage of the solar generator is exceeded and for switching on the switch element when the voltage falls below the first generator voltage or below a second generator voltage that is lower than the first generator voltage so that the input voltage range possible for the inverter circuit is increased.
In this circuit, when the intermediate circuit voltage exceeds a certain threshold value, the generator is isolated from the intermediate circuit by an additional power switch, so that the intermediate circuit and the generator only need to be set to the set threshold and semiconductors having small conduction losses may be used. Since the isolation is cancelled when the voltage falls below a second, lower voltage threshold value, the inverter remains virtually synchronized with the generator to which it is connected and can resume operation at the issue of a failure. This principle solves the problem described herein above consisting in having to devise individual semiconductors for a significantly higher voltage since the inverter needs to be connected to the idle voltage of the generator after mains failure or interruption. The circuit however does not solve the special problem consisting in that, due to installation regulations, maximum voltages must be observed in some countries, so for example 600 volt in the USA. With the circuit proposed, the idle voltage is applied to the generator each time an isolation is carried out.